It has been shown that extrusion of foamable plastic into a vacuum chamber will produce a high quality low density foam, such as the foam products sold under the trademark FOAMULAR by U.C. Industries of Parsippany, N.J. Such products are produced commercially by extruding the plastic melt into an inclined barometric leg which is in the form of a large diameter pipe which forms a vacuum chamber. The lower end of the pipe which is of substantial length projects into a pond of liquid preferably water. When the chamber is under vacuum the water is drawn from the pond at least partially to fill the chamber. The water serves to seal the chamber and cool the extrudate. The inclined configuration permits the extrudate to be extracted from the chamber on a continuous basis with the aid of a belt conveyor. Examples of such installations may be seen in the following U.S. Pat. Nos. 3,704,083; 4,044,084; 4,199,310; 4,234,529; 4,247,276; and 4,271,107.
One of the principal problems of the inclined barometric leg installations such as shown in such patents is the cost of construction. Because of the length of the leg or chamber and its inclination, the die end where the extruders are located must be at a substantial elevation above the exit or pond end. Such difference in elevation not only creates construction problems but also material handling problems during operation.
Attempts have been made to employ a horizontally extending vacuum chamber and reference may be had to U.S. Pat. Nos. 4,487,731 and 4,486,369 for an illustration of horizontal chamber vacuum extrusion systems. As seen such systems avoid a liquid pond or baffle but instead use an exit chamber which must be cycled from atmospheric to vacuum. Also the extrudate must be cut and/or stacked within the chamber which makes service and control of such equipment difficult. Also spray cooling of the product within the chamber may be inadequate for a large cross sectional size extrudate.
Other types of vacuum extrusion apparatus may be seen in U.S. Pat. Nos. 3,822,331; 1,990,434; and 2,987,768, but such are obviously not suitable for continuous foam extrusion, particularly extrudates of large cross sectional size.
It is accordingly desirble to employ a horizontal chamber which nonetheless utilizes a water baffle or seal in which the extrudate is immersed as it exits the vacuum chamber.